JPH07156819A - Front and rear wheel steering system control method - Google Patents

Abstract

(57) [Abstract] [Purpose] To provide a control method for a front and rear wheel steering device capable of improving the correction responsiveness to a side wind disturbance without giving a feeling of discomfort to a driver and improving the small-turn turning performance. A control method for a front and rear wheel steering device, wherein a front wheel steering angle is provided by steering a steering wheel and a rear wheel steering angle is provided according to a running state of a vehicle. A target yaw rate value corresponding to the steering angle is set, feedback control of the front wheel steering angle is performed based on the deviation between the target yaw rate value and the actual yaw rate value acting on the vehicle, and the rear wheel steering is adjusted so that the sideslip angle becomes zero. The corner is configured to be feedforward controlled.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for controlling a front / rear wheel steering system, and more particularly to an electronically controlled front / rear wheel steering system capable of further optimizing the response characteristics of the vehicle to disturbances on the vehicle body and changes in the running state of the vehicle. The present invention relates to a method of controlling the device.

[0002]

2. Description of the Related Art In order to improve the steering response and steering stability of a vehicle, front and rear wheel steering devices that steer both front and rear wheels have been put into practical use. This device steers the rear wheels based on the steering angle of the front wheels by steering the steering wheel and various parameters that represent the running state of the vehicle, such as the vehicle speed. This has a great effect on the improvement of the vehicle behavior when the steering wheel is steered by the driver's intention (see Japanese Patent Laid-Open No. 57-44568).

However, in the case of a front / rear wheel steering device that simply steers the rear wheels in relation to the front wheel steering angle, it is almost impossible to expect the effect of suppressing disturbance of vehicle behavior due to external factors such as side wind and road surface irregularities. There is a fact.

Therefore, in order to suppress the disturbance of the vehicle behavior due to such a disturbance, studies are being conducted to add the momentum of the vehicle to the parameters of the rear wheel steering. Recently, yaw rate is detected as the momentum of the vehicle. A control method for a front and rear wheel steering device using this as a parameter for determining a rear wheel steering angle has been put into practical use. Specifically, when it is detected that a yaw rate that does not depend on the steering angle of the steering wheel occurs in the vehicle, it is determined that this is a disturbance in the vehicle behavior due to crosswinds, and this yaw rate is automatically canceled in the direction to cancel it. It is known to perform control to steer the rear wheels.

[0005]

Generally, it is said that the smaller the yaw rate response delay, the higher the maneuverability. To improve the yaw rate responsiveness, the steering wheel steering initial stage is set. In this case, it is effective to steer the front wheels and the rear wheels in opposite directions (opposite phases). However, according to this method, the rear wheels are once steered in a direction opposite to the steering direction of the steering wheel at which the driver is about to turn, and the rear part of the vehicle swings out for a moment in a direction not intended by the driver. There is an inconvenience that the driver feels uncomfortable.

Further, when the vehicle is directed to the leeward side when a crosswind is encountered, in order to correct the direction of the vehicle to the upwind side by steering the rear wheels, the rear wheels must be steered to the leeward side. I have to
When the rear wheels are steered in this way, the rear part of the vehicle is swung to the leeward side (the side swept by the side wind), so that the momentum of the vehicle for maintaining straightness tends to increase.

On the other hand, in general, a vehicle turning at a low speed tends to face outward with respect to a tangent to the turning circle, and a vehicle turning at high speed tends to face inward to a tangent to the turning circle. In other words, the direction in which the vehicle should travel and the direction in which the vehicle is actually facing do not match, causing a deviation (this deviation angle is referred to as a sideslip angle), and this deviation affects the maneuverability.

The present invention has been devised in order to eliminate such disadvantages of the prior art, and its main purpose is to:
The responsiveness of the yaw rate can be improved without giving the driver a feeling of discomfort, and the effects of external disturbances such as side winds can be efficiently suppressed.In addition, maneuverability is improved by reducing the sideslip angle of the rear wheels. An object of the present invention is to provide a control method for a front and rear wheel steering device that can be improved.

[0009]

According to the present invention, such a purpose is to provide a front wheel steering angle by steering a steering wheel and to provide a rear wheel steering angle in accordance with a running state of a vehicle. In the control method of the front and rear wheel steering device, the target yaw rate value corresponding to the vehicle speed and the steering angle of the steering wheel is set, and the front wheel steering is performed based on the deviation between the target yaw rate value and the actual yaw rate value acting on the vehicle. This is achieved by performing feedback control of the angle and feedforward control of the rear wheel steering angle so that the sideslip angle becomes zero.

[0010]

According to this structure, the response delay of the yaw rate is compensated by increasing the front wheels in the same direction as the steering wheel steering of the driver, so that the driver does not feel uncomfortable. Also, even when a crosswind is encountered, the front wheels are steered to the windward side to correct the direction of the vehicle that is directed to the leeward side, so the behavior of the crosswind is not disturbed efficiently without swinging the vehicle body to the leeward side. Can be suppressed. Further, since the rear wheel steering angle is given so that the sideslip angle becomes zero, the trajectory of the turning circle and the actual direction of the vehicle coincide with each other, and the maneuverability is improved.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The structure of the present invention will be described in detail below with reference to specific embodiments shown in the accompanying drawings.

FIG. 1 schematically shows the entire structure of a front and rear wheel steering device for a vehicle to which the present invention is applied. A steering shaft 2 having a steering wheel 1 fixed at one end is connected to a steering rod 3 of a front wheel steering device FS via a steering gear box 4 having a rack and pinion mechanism and the like built therein. Both ends of the steered rod 3 are connected to respective knuckle arms 6 supporting the left and right front wheels 5 via tie rods 7, respectively. Further, the steering gear box 4 can be moved in the vehicle width direction by an electric motor 8 fixed to the vehicle body, and a compensation front wheel steering angle is given if necessary. It should be noted that the steering gear box 4 has a built-in electric motor to generate an auxiliary torque for the steering force of the steering wheel 1.

Rear wheel steering device RS arranged at the rear of the vehicle
Is a steering rod 10 extending in the vehicle width direction and a motor 11
It is designed to be driven by. Both ends of the steered rod 10 are the same as the steered rod 3 on the front wheel 5 side.
The knuckle arms 13 that support 2 are respectively connected via tie rods 14.

The front wheel steering system FS is equipped with a steering angle sensor 15 for measuring the compensation steering amount of the front wheels 5, and the rear wheel steering system RS is for measuring the steering amount of the rear wheels 12. To
The rudder angle sensor 16 is attached. A steering angle sensor 17 for detecting the steering amount of the steering wheel 1 is attached to the steering shaft 2. In addition to this, a vehicle speed sensor 18 is provided on each of the wheels 5 and 12, and the yaw rate sensor 1 is provided at an appropriate position on the vehicle body.
9 is provided.

Each of these sensors 15 to 19 includes an electric motor 8 of the front wheel steering device FS and an electric motor 1 of the rear wheel steering device RS.
1 is electrically connected to a computer unit 20 which controls driving.

In this device, the steering wheel 1
When the driver steers, the steering rod 3 of the front wheel steering device FS is mechanically driven to steer the front wheels 5. At the same time, the outputs of the sensors 15 to 19 are input to the computer unit 20, respectively. And each of these sensors 1
The computer unit 20 determines the compensation turning amount of the front wheels 5 and the optimum turning amount of the rear wheels 12 based on the respective input values of the steering wheel steering angle θh, the vehicle speed V, and the yaw rate γ emitted by the vehicle wheels 5 to 19. The electric motor 8 of the steering device FS and the electric motor 11 of the rear wheel steering device RS are driven to steer the front wheels 5 and the rear wheels 12.

FIG. 2 shows a schematic block diagram of the control of the front and rear wheel steering device having the above-mentioned configuration. First, the compensation front wheel steering angle control will be described. When the steering wheel steering angle θh is input, the steering gear box 4 of the front wheel steering device FS
The reference front wheel steering angle δf · conv is determined by the gear ratio of. On the other hand, the target yaw rate value γr for the steering wheel steering angle θh at this time is calculated by the target yaw rate calculation unit 22. The vehicle speed V is also input to the target yaw rate calculation unit 22, and an appropriate target yaw rate value γr considering the traveling speed is calculated. In the target yaw rate calculation unit 22, an early yaw rate is generated as a target value in order to compensate for a response delay from the time when the steering wheel 1 is steered until the yaw rate γ actually occurs in the vehicle 23. It has become.

The target yaw rate value γr is compared with the actual yaw rate value γ of the vehicle 23 to obtain the yaw rate deviation γe (γe = γr-γ). And this yaw rate deviation γe
Is input to the feedback controller 24, and the compensation front wheel steering angle δf · act for canceling the yaw rate deviation γe is input.
To calculate. Then, the compensation front wheel steering angle δf · act is converted to the reference front wheel steering angle δf · act by steering the steering wheel 1.
a form added to conv (δf = δf · conv
The front wheels 5 are steered at + δf · act). In addition, the vehicle speed V is also input to the feedback controller 24, and an appropriate feedback control in consideration of the traveling speed is performed.

By the way, it is difficult to grasp the vehicle behavior in the low speed range, and it is difficult to set proper control parameters. Especially, the behavior of the feedback control system may become unstable. Further, in the low speed range, the transient characteristics of the vehicle behavior are weak and the influence on the driver is small, so there is little need to compensate the response delay.

Therefore, in the present invention, a coefficient for adjusting the gain of the feedback controller 24 is set and changed according to the vehicle speed V to suppress the feedback control amount in the low speed range, and the vehicle behavior becomes unsatisfactory. I try to suppress it from becoming stable. Specifically, the transfer function Cp (s) of the feedback controller 24 is multiplied by a gain adjustment coefficient Kp as shown in FIG. The coefficient Kp is a function of the vehicle speed V, and is set to 0 or an extremely small value in the low speed range, the value increases as the vehicle speed V increases, and the value becomes 1 above a certain vehicle speed. As a result, the feedback element in the low speed range is reduced to 0 or extremely small, which prevents the vehicle behavior from being adversely affected. Then, the feedback element becomes large as the vehicle speed V increases, and the feedback element becomes sufficiently effective in the high speed range where the improvement of the responsiveness and the disturbance suppressing effect are important.

On the other hand, the target yaw rate value γr is set including a transient element with respect to the steering angle input θh of the steering wheel 1. Since this is generally set by a linear model, the steering angle of the steering wheel θh
The target yaw rate value γr increases as the steering angle θh increases. However, since the motion of the vehicle depends on the frictional force between the road surface and the tire, it is not possible to realize motion in a range exceeding the limit of the road surface gripping force of the tire. Therefore, if the rudder angle is controlled based on the yaw rate deviation γe calculated by ignoring the road surface condition, there is a possibility that the rudder angle control that attempts to make a turn exceeding the road surface grasping force of the tire will be performed, and the vehicle behavior It can lead to instability.

Therefore, in order to deal with the above problem, before the limiter set in the target yaw rate calculation unit 22 is set so as to generate with an appropriate transfer function Gm (s) with respect to the steering wheel steering angle θh. From the target yaw rate output γri and the vehicle speed V, the lateral acceleration αi corresponding to these is calculated by the following formula: αi = V · γri The frictional force limit between the road surface and the tire is determined by the friction coefficient μ, so the calculated lateral acceleration αi Therefore, the limited lateral acceleration αo on the table is set so as not to exceed the friction coefficient μ.

The yaw rate γro corresponding to the limited lateral acceleration αo converted by this limiter table is given by the following equation. γro = αo / V By setting this value γro as the target yaw rate value γr output from the target yaw rate calculation unit 22, it is possible to prevent the target yaw rate value from becoming excessive without actually measuring the lateral acceleration.

An example of the limiter table is shown in FIG. By setting the limiter table, the steering characteristic of the vehicle can be adjusted. Specifically, for example, if the limiter characteristic of αi = αo shown by the dotted line in FIG.
There is a neutral steer tendency, and there is an understeer tendency as the distance from αi = αo increases. In actual driving, it is better to maintain neutral steering characteristics up to a higher lateral acceleration range at medium speeds, which makes it easier to drive on curved roads, but at high speeds, it is better to have some understeer tendency. The stability is increased and the tension given to the driver is alleviated. Therefore, by changing the characteristic of the limiter table according to the vehicle speed V, the neutral steer characteristic is obtained in the low speed running range,
Understeer characteristics can be achieved in the high-speed driving range. In addition to the vehicle speed, the vehicle state changes depending on the loading state and the longitudinal acceleration, so it is desirable to be able to deal with these.

By the way, the friction coefficient μ of the road surface is not constant, and particularly decreases on the ice and snow road surface. Therefore, if the friction coefficient μ, which is the reference for setting the limiter table, is a fixed value, excessive steering is performed when the actual road surface is below the reference friction coefficient, resulting in unstable vehicle behavior. It can happen. On the other hand, when the road surface has a friction coefficient higher than the upper limit value of the limiter table, turning is suppressed even though there is a margin, and this also leads to vehicle behavior that does not suit the driver. Become.

Therefore, in the present invention, the road surface friction coefficient μ is actually measured and is set as the upper limit value of the limiter table.
Specifically, the actual road surface friction coefficient μ is detected by the road surface friction coefficient detecting means, and this is input to the limiter circuit of the target yaw rate calculation unit 22 to change the upper limit value of the limiter table according to the road surface condition. . As a result, in the dry road, the road surface friction coefficient μd corresponding to the dry road shown in FIG.
The high upper limit is taken as shown by the solid line in Fig. 5 and, in the case of ice and snow roads, the upper limit is lowered as shown by the dotted line in Fig. 5 in accordance with the road surface friction coefficient μw of the ice and snow road. A target yaw rate value γr can be calculated.

It should be noted that the road surface friction coefficient μ may be evaluated stepwise on the slipperiness of the road surface without changing continuously, and the upper limit value of the limiter table may be changed stepwise according to it. (See FIG. 6).

Next, the sideslip angle control will be described.

The compensation front wheel steering angle δf · act based on the deviation γe between the target yaw rate value γr and the actual yaw rate value γ.
The addition control of is effective in improving the response of the yaw rate, but is not effective in optimizing the orientation of the vehicle when turning. Therefore, the front wheel steering angle δf and the vehicle speed V are calculated by the rear wheel steering angle calculation unit 25.
Is input to calculate an appropriate rear wheel steering angle δr.

When the transfer function of the front wheel sideslip angle is Hf (s) and the transfer function of the rear wheels sideslip angle is Hr (s), the relationship between the front wheel steering angle δf and the rear wheel steering angle δr and the sideslip angle β is as follows. As shown in FIG. 7, that is, given by the following equation. β = δf · Hf (s) + δr · Hr (s)

Therefore, in order to make the sideslip angle β zero, the following equation is established and δr =-{Hf (s) / Hr (s)} δf From this, it is possible to realize by the control shown in FIG. It turns out that

Transfer function Cq (s) of the rear wheel steering angle calculation unit 25
Is expressed as follows when the motion of the vehicle is expressed by a two-wheel model.

[0033]

[Equation 1]

According to the above equation, the sideslip angle β may occur in a transient state (such as when a fast steering wheel is operated). This is partly because the dynamic characteristics of the tire are not taken into consideration. Therefore, the function is determined as shown below.

[0035]

[Equation 2]

Thus, by controlling the rear wheel steering angle δr in accordance with the front wheel steering angle δf, the sideslip angle is suppressed,
The maneuverability can be improved.

By the way, the steady gain of the transfer function Cq (s) of the rear wheel steering angle calculation unit 25 (the ratio between the front wheel steering angle δf and the rear wheel steering angle δr in the steady state) is as shown in FIG. The steering angle ratio is set to be large in the low speed range. However, in an actual vehicle, the maximum steering angle of the rear wheel 12 is limited for the sake of ensuring safety at the time of failure and avoiding interference with the wheel house or suspension arm. Therefore, the rear wheel steering angle given as the control signal may not be actually realized.

Therefore, a coefficient Kq for adjusting the steady-state gain of the rear wheel steering angle calculation unit 25 is set and changed according to the vehicle speed V to issue a rear wheel steering angle command exceeding the limit value in the low speed range. I try to prevent that. If the gain of the rear wheel steering angle δr is reduced here, the side slip angle suppressing effect during turning is lost, but the side slip angle has a small effect on the driver in the low speed range, and there is almost no problem. Also, the small turning performance is improved as compared with a normal vehicle. Specifically, the transfer function Cq (s) of the rear wheel rudder angle calculation unit 25 is calculated as shown in FIG.
Multiply by the coefficient Kq as shown in 0. This coefficient Kq is a function of the vehicle speed V and takes a value smaller than 1 in the low speed range,
The value increases as the vehicle speed V increases, and is set to 1 above a certain vehicle speed. As a result, the steady gain has the characteristic shown in FIG. 11, and the steering angle ratio in the low speed range is suppressed to a range that does not exceed the maximum steering angle of the rear wheels 12.

Now, when only the yaw rate feedback control of the front wheels is executed (A), when only the rear wheel control is executed so that the sideslip angle becomes zero (B), and both of them are controlled in combination. When the case (C) is compared with the case where only the conventional front wheels are steered (2WS), the phase lag of the yaw rate in the frequency response has the result shown in Table 1, which is a control method of the present invention. Control (C)
Gave the best results.

[0040]

[Table 1]

In the comparison between only the yaw rate feedback control of the front wheels (A) and the combined control (C), the necessary corrected steering amount of the front wheels shows that only the yaw rate feedback control of the front wheels (A) is obtained according to the combined control (C). About 1 compared to
Considering a mechanism for moving the steering gear box 4 in the vehicle width direction, advantages such as downsizing of the moving mechanism due to reduction of the required displacement amount and facilitation of failsafe can be obtained.

In the comparison between only the sideslip angle zero control (B) and the composite control (C), when an attempt is made to enhance the response of the yaw rate at a certain vehicle speed at which the steady sideslip angle is inward,
According to only the sideslip angle zero control (B), the reverse input is reversed by the step input, so that the rear portion of the vehicle body is swung outward, which causes discomfort, and the lateral movement of the vehicle is not particularly fast. In the case of combined control (C), there is no particular difference in the rear wheel steering itself, but the front wheel steering angle is increased by yaw rate feedback and the front part of the vehicle body enters inside, so the flow feeling at the rear part of the vehicle body is improved. Since it disappears, quick lateral movement can be performed without feeling any discomfort.

[0043]

As described above, according to the present invention, the front wheel is controlled to be increased based on the comparison between the actual yaw rate and the target yaw rate to compensate for the response delay of the yaw rate, so that the driver does not feel uncomfortable. Therefore, since the feeling of flow at the rear is reduced, the driver does not feel uncomfortable, and when a crosswind is encountered, the front wheels are steered to the windward side of the vehicle, which is directed to the leeward side. Since the direction is corrected, the disturbance of the behavior due to the side wind can be efficiently suppressed without swinging the vehicle body to the leeward side. Further, since the rear wheel steering angle is given so that the sideslip angle becomes zero, the trajectory of the turning circle and the actual direction of the vehicle coincide with each other, and the maneuverability is improved.

[Brief description of drawings]

FIG. 1 is an overall configuration diagram of a vehicle with front and rear wheel steering devices to which the present invention is applied.

FIG. 2 is a block diagram of a control law of the present invention.

FIG. 3 is a gain adjustment coefficient diagram with respect to vehicle speed.

FIG. 4 is a characteristic diagram showing an example of a limiter table.

FIG. 5 is a characteristic diagram showing another example of a limiter table.

FIG. 6 is a characteristic diagram showing still another example of a limiter table.

FIG. 7 is a block diagram showing a relationship between a front wheel steering angle and a rear wheel steering angle and a sideslip angle.

FIG. 8 is a block diagram of zero sideslip angle control.

FIG. 9 is a steady steering angle ratio diagram with respect to the vehicle speed before correction.

FIG. 10 is a gain adjustment coefficient diagram with respect to vehicle speed.

FIG. 11 is a steady steering angle ratio diagram with respect to the corrected vehicle speed.

[Explanation of symbols]

 1 Steering Wheel 2 Steering Shaft 3 ・ 10 Steering Rod 4 Steering Gear Box 5 Front Wheel 6 ・ 13 Knuckle Arm 7 ・ 14 Tie Rod 8 ・ 11 Electric Motor 12 Rear Wheel 15 ・ 16 ・ 17 Steering Angle Sensor 18 Vehicle Speed Sensor 19 Yaw Rate Sensor 20 Computer Unit 22 Target yaw rate calculation unit 23 Vehicle 24 Feedback controller 25 Rear wheel steering angle calculation unit

Continuation of the front page (51) Int.Cl. ⁶ Identification number Office reference number FI technical display location B62D 113: 00 137: 00

Claims (7)

[Claims] 1. A method for controlling a front and rear wheel steering device, wherein a front wheel steering angle is provided by steering a steering wheel, and a rear wheel steering angle is provided according to a running state of the vehicle. A target yaw rate value corresponding to the steering angle of the steering wheel is set, and the front wheel steering angle is feedback-controlled based on the deviation between the target yaw rate value and the actual yaw rate value acting on the vehicle, and the sideslip angle becomes zero. A method for controlling a front / rear wheel steering device, characterized in that the rear wheel steering angle is feed-forward controlled. 2. The control method for the front and rear wheel steering system according to claim 1, wherein the transfer function used for the feedforward control of the rear wheel steering angle is determined in consideration of the lateral rigidity of the tire. . 3. The front and rear wheel steering system according to claim 1, wherein a parameter for adjusting the gain of the feedforward control of the rear wheel steering angle is set and the parameter is changed according to the vehicle speed. Control method. 4. The control method for a front / rear wheel steering system according to claim 1, wherein the target yaw rate value is limited based on a preset limiter table. 5. The limiter table comprises a table in which a relationship between a lateral acceleration value calculated based on a vehicle speed and a yaw rate target value before applying the limiter and a lateral acceleration value limited in advance is set. The control method for the front and rear wheel steering device according to claim 4. 6. The method for controlling a front / rear wheel steering system according to claim 4, wherein the characteristic of the limiter table is changed according to a measured value of a friction coefficient of a road surface. 7. The characteristic of the limiter table is set to a vehicle speed,
The method for controlling the front and rear wheel steering device according to claim 4 or 5, wherein the method is changed according to a vehicle state such as a vehicle weight.